After can formation by cold forming, aluminum fines, lubricating oils and other contaminants remain on the surface. It is necessary to clean the surface thoroughly prior to the further treatment which often includes the application of one or more surface coatings.
Early cleaning compositions for aluminum surfaces proposed the use of aqueous acidic compositions either alone or with added fluoride at temperatures in the range of 185.degree. to 200.degree. F. (85.degree. to 93.degree. C.). In U.S. Pat. No. 4,009,115 and its Re-Issue No. 32,661, Binns proposed the addition of 0.05 to 0.1 grams per liter of hydrofluoric acid as a means of reducing the processing temperature to within the range of 90.degree. to 135.degree. F. (32.degree. to 57.degree. C.). The compositions proposed by Binns preferably contain 0.1 to 10 grams per liter of a surfactant which could be anionic, cationic or nonionic.
King et al. in U.S. Pat. No. 3,969,135 proposed the use of an aqueous acidic cleaning composition containing a blend of two surfactants. This composition was also suitable for relatively low temperature use and preferably contained 0.01 to 0.4 weight percent of a fluoride accelerator. The surfactants proposed by King et al. (hereinafter abbreviated as simply "King") were a polyalkoxylated straight chain alcohol and a polyalkylene glycol-abietic acid surfactant. The King compositions are currently used in one or more, preferably at least two, cleaning stages in the commercial production of aluminum cans, but not usually in the first stage. Normal current practice is to provide most of the acidity in the precleaning stage, immediately before washing the cans with a cleaner composition as taught by King, by directing part of the acid cleaner washing solution into the pre-cleaner washing solution, which otherwise consists largely of tap water, with optional additions of acid. When a second stage acid washing solution as taught by King is used in this manner, the ratio between the two kinds of surfactants normally will be the same in the fast stage as in the second, if nothing else is added to the fast stage solution, inasmuch as no preferential dragout of one type of surfactant has been observed.
Washing operations with solutions that include nonionic surfactants are normally conducted at or slightly below the cloud point of the washing solution, which is the temperature at which the surfactant comes out of or goes into aqueous solution with changing temperature. It is a characteristic of most nonionic surfactants, including those taught by King as noted above, that they become less soluble in water as the temperature of an aqueous surfactant composition is raised. At temperatures significantly below the cloud point of a particular composition, foaming of the aqueous solution generally occurs quite easily, and it is desirable to avoid foaming in the present washing operations. If the temperature of the composition is too far above the cloud point, separation of the surfactant from the aqueous medium occurs and leads to a loss of detergent ability and a loss of the actual surfactant material. Accordingly, the present washing operations are normally conducted at or below the cloud point where the detergent ability of the composition is still effective and foaming can be minimized.